From their introduction:
"Strategic unraveling in our model proceeds as follows. There is a loss in efficiency when some agents go early: Information about the quality of the matches arrives late, so it is better for efficiency to wait until the information has arrived to make a match. If some agents go early anyway, this forces later matches to be less efficient. The result is a negative externality that makes it more tempting for all agents to go early. It may push some additional agents over the threshold by which they decide to go early. In turn, these additional agents going early makes it even more tempting to go early|and so on and so forth."